one also makes the Exchange servers authoritative for the
"exchange.example.com" domain, but forward anything unresolved int the
parent domain (example.com) to the cross-domain mail hub for routing.

This is most useful when multiple mail store environments are present.
(Multiple Exchange installations, and/or other non MSFT mailstores).

Some people find it easier to rewrite mailbox->primary mail in
smtp_generic_maps, so that the backend servers still see the primary
address, but the Postfix queue sees mailbox domains, and thereby avoid
per-user transport lookups, which typically involve much slower
(compared to indexed-file lookups) LDAP or SQL queries and can adversely
impact queue manager performance.